A semiconductor memory device may be a memory device that is implemented with semiconductor materials such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), and/or indium phosphide (InP). Semiconductor memory devices may be one of a volatile memory device and a nonvolatile memory device.
The volatile memory device may be a memory device in which stored data may be erased when a power source is shut off. Examples of volatile memory devices include Static Random Access Memory (SRAM), Dynamic Random Access Memory (DRAM), and Synchronous Dynamic Random Access Memory (SDRAM). A nonvolatile memory device may be a memory device that retains stored data even when a power source is shut off Examples of nonvolatile memory devices include Read-Only Memory (ROM), Programmable Read Only Memory (PROM), Erasable Programmable Read Only Memory (EPROM), Electrical Erasable Programmable Read Only Memory (EEPROM), flash memory devices, Phase-change Random Access Memory (PRAM), Magnetoresistive Random Access Memory (MRAM), Resistive Random Access Memory (RRAM) and Ferroelectric Random Access Memory (FRAM). Flash memory devices may be largely categorized into a NOR type and a NAND type.
Semiconductor memory devices having a three-dimensional structure have been studied to increase integration of semiconductor devices.